It is known to mount image capture devices, such as, for example, digital or analogue video cameras, on a motor vehicle in order to produce a video image of an aspect of the environment exterior of the vehicle. For example, in order to assist in parking and manoeuvring a vehicle in confined spaces, it is known to mount such image capturing devices on respective opposite sides of the vehicle, for example, on side rear view mirror housings which extend sidewardly from the driver and front passenger doors of the vehicle. The image capture devices are mounted in the side rear view mirror housings with the field of view of the image capture devices directed downwardly towards the ground for capturing plan view images of the ground on respective opposite sides of the vehicle adjacent the vehicle. Typically, a visual display unit is located in the vehicle, either in or on the dashboard, or in a location corresponding to that of a conventional interiorly mounted rear view mirror. When a driver is undertaking a parking manoeuvre or a manoeuvre in a confined space, a plan view image of the vehicle with the respective plan view images of the ground on respective opposite sides of the vehicle can be displayed on the visual display unit. The plan view display of the vehicle and the ground on respective opposite sides of the vehicle assists the driver in parking, and in particular, carrying out a parking manoeuvre for parking the vehicle in a parking space parallel to a kerb of a footpath or the like.
However, in order that the plan view images of the ground accurately reflect the positions of objects relative to the vehicle, which are captured in the images, it is essential that the plan view images of the ground juxtapositioned with the plan view image of the vehicle should accurately represent a top plan view of the ground adjacent the respective opposite sides of the vehicle exactly as would be seen when viewed from above. In other words, the edges of the respective plan view images of the ground which extend along the sides of the plan view image of the vehicle must correspond directly with the edge of the ground along the sides of the vehicle when viewed in plan view from a position above the vehicle. Otherwise, the positions of objects in the respective plan view images of the ground will not be accurately positioned relative to the vehicle. For example, if the edge of one of the plan view images of the ground adjacent the corresponding side of the plan view image of the vehicle corresponds with a portion of a plan view of the ground which is spaced apart from the side of the vehicle, then the positions of objects in the plan view image of the ground will appear closer to the vehicle in the image than they actually are. Conversely, if one of the image capture devices is mounted on a side mirror housing so that an image of a portion of the ground beneath a side of the vehicle is captured, the positions of objects captured in the plan view image will appear farther away from the vehicle than they actually are, with disastrous results, particularly if a driver is parking the vehicle parallel to a wall or bollards.
Accordingly, it is essential that the plan view images of the ground when displayed on the visual display screen juxtapositioned along with the plan view image of the vehicle must be representative of plan views of the ground on respective opposite sides of the vehicle exactly as would be seen from a top plan view of the vehicle and adjacent ground. In order to achieve such accuracy, the image capture devices would have to be precision mounted on the vehicle. In practice this is not possible.
Accordingly, in order to achieve the appropriate degree of exactness and accuracy of the plan view images of the ground relative to the plan view image of the vehicle, it is necessary to calibrate the outputs of the image capture devices. Calibration values determined during calibration of the image capture devices are then used to correct subsequently captured image frames for offset of the image capture devices from ideal positions thereof, so that plan view images of the ground subsequently outputted for display with the plan view image of the vehicle are exact representations of the ground on respective opposite sides of the vehicle. Such calibration can be accurately carried out in a factory during production of the motor vehicle. Typically, the image capture devices are relatively accurately fitted in the side mirror housings of the motor vehicle, and by using suitable grid patterns on the ground, calibration can be effected. However, the environments in which motor vehicles must operate are generally relatively harsh environments, in that side mirror housings are vulnerable to impacts with other vehicles or stationary objects. While such impacts may not render the orientation of the side mirror housing unsuitable for producing an adequate rear view from a rear view mirror mounted therein, such impacts can and in general do result in the image capturing device mounted therein being knocked out of alignment, in other words, being offset from its ideal position. Additionally, where a vehicle is involved in a crash, or alternatively, where a side mirror housing requires replacement, re-calibration of the image capture device refitted in the new side mirror housing will be required. Such re-calibration, which typically would be carried out using a grid pattern on the ground, is unsatisfactory, since in general, it is impossible to accurately align the vehicle with the grid pattern in order to adequately calibrate the image capture device, unless the calibration is being carried out under factory conditions.
There is therefore a need for a method and a calibration system for calibrating an output of an image capture device mounted on a vehicle to compensate for offset of the image capture device from an ideal position which addresses these problems.
The present invention is directed towards such a method and a calibration system. The invention is also directed towards a method and an image vision system for producing an output image frame from a captured image frame captured by an image capture device with the output image frame with compensation for offset of the image capture device from an ideal position. The invention also relates to a vehicle comprising a calibration system for calibrating an output of an image capture device mounted on the vehicle to compensate for offset of the image capture device from an ideal position, and the invention further relates to a vehicle comprising an image capture device mounted thereon and an associated image vision system for producing an output image frame from a captured image frame captured by the image capture device with the output image frame with compensation for offset of the image capture device from an ideal position.